Assessment of the acidification and changes of the chemical contents of seasonal snow within the snowmelt process were carried out at a small watershed of the northern Hokkaido from 194 to 1996 include two snow cover seasons. Vegetations at the watershed are mixed forests of evergreen needle-leaf trees and deciduous broad-leaved trees with bamboo bush undergrowth. The surface soil is composed primarily of silt and clay, and is underlain to a depth of 3m by tuffaceous silt stone and sandstone of the Neocene. Snow samples were taken from three fixes sites : open site, forest fllor, forest canopy. Water samples were obtained from the bottom of the snow cover, from 10cm deep and from the stream. The range of pH in the dry snow was 4.7 to 5.3. These values were similar to the snow accumulated on the forest floor and the canopy. Dry snow has homogenous distributions of pH,EC and chemical constituents in the lateral direction of each layred structure. However, once melt water penetrated, the homogenous distributions were partly destroyed even in winter. During the snowmelt period pH of the snow increased and EC decreased to below 10muS/cm. pH of the melt water that percolated through 10cm deep soil accounted for 6.4 to 5.8, and those of the stream 7.1 to 6.8. The values did not change much through the snowy season. The percolated water showed high concentrations of anion compared with those of snow, but the water contained higher cation, which neutralized the anion, and pH of the water increased.